1. Field of the Invention
The present invention relates to a thermoelectric conversion device, a radiation detector, and a radiation detection method that utilize an anisotropic thermoelectric effect.
2. Description of Related Art
When a temperature difference occurs between both ends of a thermoelectric conversion material, an electromotive force (a thermal electromotive force) is generated in proportion to the temperature difference. The phenomenon that thermal energy is converted into electrical energy in a thermoelectric conversion material is known as the Seebeck effect. The generated electromotive force V is expressed as V=SΔT, where ΔT is a temperature difference and S is a Seebeck coefficient peculiar to the material.
Generally, in a thermoelectric conversion material that exhibits isotropic physical properties, an electromotive force is generated by the Seebeck effect only in the same direction as the direction in which the temperature difference has occurred. In contrast, in a thermoelectric conversion material that exhibits anisotropy in its electrical transport properties, an electromotive force is generated in the direction orthogonal to the direction in which the temperature difference has occurred, due to the inclined arrangement of the crystal axes of the material. The electrical transport properties are the behavior of electrically-charged electrons and positive holes that move in a substance. As described above, the phenomenon that due to the inclined arrangement of the crystal axes of the material, an electromotive force is generated in the direction different from the direction in which the temperature difference has occurred (a heat flow direction) is referred to as an anisotropic thermoelectric effect or an off-diagonal thermoelectric effect.
FIG. 10 is a diagram of a coordinate system for explaining the anisotropic thermoelectric effect. As shown in FIG. 10, the crystal axes abc of a sample 100 are inclined to the spatial axes xyz. In the sample 100, when a temperature difference ΔTz is applied in the direction along the z axis, an electromotive force Vx is generated in the direction orthogonal to the z axis, i.e. the direction along the x axis. The electromotive force Vx is represented by Formula (1):
[Mathematical Formula 1]
                              V          x                =                              1                          2              ⅆ                                ⁢          Δ          ⁢                                          ⁢                                    T              z                        ·            Δ                    ⁢                                          ⁢                      S            ·            sin                    ⁢                                          ⁢          2          ⁢                                          ⁢          α                                    (        1        )            
where l denotes the width of the sample 100, d denotes the thickness of the sample 100, α denotes the inclination angle of the a-b plane to the surface (the x-y plane) of the sample 100, and ΔS denotes the difference (the difference that occurs due to anisotropy) between the Seebeck coefficient Sc in the c-axis direction and the Seebeck coefficient Sab in the a-b in-plane direction.
As shown above, the anisotropic thermoelectric effect is obtained in a thermoelectric conversion material in which the crystal planes serving as a factor inducing anisotropy thereof (hereinafter referred to simply as “crystal planes” corresponding to the a-b planes in the sample 100 shown in FIG. 10) are arranged inclined to the surface of the thermoelectric conversion material (corresponding to the surface (x-y plane) of the sample 100 shown in FIG. 10).
Conventionally, there has been proposed, as a thermoelectric conversion device that utilizes this effect, a thermoelectric conversion device in which a thin film (hereinafter referred to as an “inclined thin film”) made of a thermoelectric conversion material is disposed on a substrate and the crystal planes of the material are arranged inclined to the substrate plane (the main surface of the substrate). It has also been proposed to use a thermoelectric conversion device having this configuration as a radiation detector (see, for example, JP 08 (1996)-247851 A).
FIG. 11 shows the basic structure of a thermoelectric conversion device that utilizes the anisotropic thermoelectric effect. This thermoelectric conversion device comprises an inclined substrate 111, an inclined thin film 112 disposed on the inclined substrate 111, and a pair of electrodes 113, 114 disposed on the inclined thin film 112. The inclined substrate means a substrate in which the low-index planes (planes 116 in the inclined substrate 111 shown in FIG. 11) of a crystal as a material that constitutes the substrate are inclined to the substrate plane. The inclined thin film is the one as described above, and in the basic structure shown in FIG. 11, the crystal planes 115 of the inclined thin film 112 are inclined to the surface of the inclined substrate 111. In FIG. 11, the referential sign 119 indicates a direction in which the crystal planes 115 are aligned in an inclined manner, and the referential sign 120 indicates a direction in which the planes 116 of the inclined substrate 111 are aligned in an inclined manner.
In a radiation detector that utilizes a thermoelectric conversion device having such a configuration, when an incident electromagnetic wave enters the radiation detector through the front surface of the inclined thin film 112, a temperature gradient 117 occurs in the direction from the front surface of the inclined thin film 112 to the back surface thereof in the radiation detector. As a result, an electromotive force is generated in the direction 118 parallel to the surface of the inclined thin film 112 by the above-mentioned anisotropic thermoelectric effect. Reading the electromotive force thus generated allows the electromagnetic wave to be detected.
However, conventional techniques have not achieved a sufficiently high sensitivity for practical use of radiation detectors.
One of the problems to be solved to increase the sensitivity of this type of radiation detectors is that the energy of the incident electromagnetic wave is not utilized (i.e., the energy is not converted into a thermal electromotive force) effectively. The proportion of the incident electromagnetic wave energy converted into a thermal electromotive force depends on the proportion of the electromagnetic wave absorbed in the inclined thin film. The thickness of the inclined thin film can be increased to increase the absorption of the electromagnetic wave in the inclined thin film. However, according to the above Formula (1), when the temperature difference is constant, the electromotive force that is generated by the anisotropic thermoelectric effect decreases as the thickness of the inclined thin film increases. Therefore, it is desirable to reduce the thickness of the inclined thin film in order to increase the sensitivity of the radiation detector. However, if the thickness is reduced, the proportion of the electromagnetic wave absorbed in the inclined thin film decreases, and thus the electromagnetic wave energy is not converted effectively into a thermal electromotive force. This means that techniques for converting the incident electromagnetic wave more effectively into a thermal electromotive force are necessary to increase the sensitivity of the radiation detector.